In monochrome copiers and laser beam printers (also abbreviated below as “LBP”), single-component development systems that use magnetic toners are in wide use due largely to advantages in terms of cost and simplicity of device construction. A variety of investigations are currently underway, both on the toner and the apparatus itself, which are aimed at achieving even higher image quality in such monochrome copiers and LBPs. To enhance the image quality in monochrome copiers and LBPs, the approaches taken from the standpoint of the toner have included reducing the diameter, sharpening the size distribution and increasing the circularity of the toner particles.
When the toner has a smaller particle diameter, the resolution increases, enabling a high precision image to be obtained. When the toner has a sharp particle size distribution, the charge distribution becomes uniform, the behavior of the toner becomes uniform in the development and transfer steps, and toner which lowers image quality by, for example, jumping to non-image areas decreases.
Also, if the unevenness at the surface of toner that has been rendered highly circular could be reduced, it would be possible to uniformly add to the toner surface various external additives having the function of imparting the toner with a charging performance. This would make the charge distribution of the overall toner more uniform, thereby enhancing, as noted above, the image quality. In addition, the toner flowability would increase, improving the rise in electrostatic charge and enabling high-quality images to be obtained from the start of printing.
Where toners having a high circularity and a sharp particle size distribution were once manufactured primarily by conventional pulverization processes, such toners are today increasingly being manufactured by polymerization processes, by emulsion aggregation processes or by using a hot air current to spheronize toner particles. However, when a toner having a high circularity is not transferred and remains on the electrostatic latent image bearing member, the toner has a poor recoverability from the electrostatic latent image bearing member, so-called cleaning performance, which tends to give rise to image defects caused by faulty cleaning.
To address this problem, Patent Documents 1 and 2 adjust the amount of external additive and the external addition conditions with the aim of controlling the dynamic friction coefficients of the toner and the electrostatic latent image bearing member, Patent Document 3 suppresses adhesion between the toner and the electrostatic latent image bearing member by adding external additive having a large particle diameter, and Patent Document 4 controls the friction coefficient of the toner surface by adjusting the crystallinity of the binder resin.
However, although such approaches focused on various indicators are expected to provide some degree of improvement, because the cleaning stability during long-term use under harsh conditions such as a low-temperature environment is inadequate, a fundamental solution to the above problem is not achieved. Hence, a radical approach focused on more fundamental indicators of faulty cleaning is required.